1. Field of the Invention
The present invention generally relates to a liquid crystal display device and a method for manufacturing the same, and more specifically to a liquid crystal display device comprising upper and lower substrates facing each other, liquid crystal interposed at a space between the upper and lower substrates, and counter electrode and pixel electrode formed on the lower substrate for driving the liquid crystal and a method for manufacturing the same.
2. Description of the Related Art
Recently, liquid crystal display devices(hereinafter abbreviated as LCDs) are widely utilized in word processors, personal computers, projection televisions and miniaturized televisions.
These LCDs are divided into various modes according to their liquid crystal molecule arrangements. The In-Plane Switching(IPS) mode which is applied to a super TFT-LCD and developed by Hitachi in 1995 was suggested to overcome the narrow viewing angle and the low contrast ratio of the twisted nematic(TN)-LCDs.
A description of IPS-LCDs is given below with reference to FIG. 1.
FIG. 1 shows a unit cell region of a LCD for convenience. First, a gate bus line 101 is arranged along a X-direction shown in FIG. 1. Next a data bus line 105 is arranged along a Y-direction to perpendicularly intersect with the gate bus line 101.
Near the intersection of the gate bus line 101 and the data bus line 105, a thin film transistor(hereinafter abbreviated as "TFT") including a channel layer 104 is provided. The channel layer 104 is formed on a selected portion of the gate bus line 101, and the data bus line 105 is overlapped with one end of the channel layer 104.
Inside the unit cell domain defined by the gate bus line 101 and the data bus line 105, a counter electrode 102 is formed in a rectangular frame shape. The counter electrode 102 is electrically connected to other counter electrodes which are adjacent to neighboring unit cell domains. A pixel electrode 106 comprises portions overlapped with the counter electrode 102 that are parallel to the gate bus line 101, and a portion parallel to the data bus line 105 which divides a space surrounded by counter electrodes 102. Also, the pixel electrode 106 is overlapped with the other end of the channel layer 104.
On an upper substrate(not shown) facing the lower substrate 100, color filters(not shown) and black matrix(not shown) dividing the color filters are formed. Next, a liquid crystal material is interposed between the lower substrate 100 and the upper substrate (not shown). The liquid crystal is driven by a field formed between the counter electrode 102 and the pixel electrode 106. The gate bus line 101, data bus line 105, counter electrode 102, and the pixel electrode 106 are all made from metal.
The IPS-LCDs having a structure as described above have counter electrodes 102 and pixel electrodes 106 driving the liquid crystal all formed in the lower substrate 100. Therefore, if a selected voltage is applied to the counter electrode 102 and the pixel electrode 106, a field horizontal to the lower substrate 100 is created. If the liquid crystal molecules are of positive dielectric anistropy (.DELTA..epsilon.&gt;0), they also lie parallel to the field. Since the liquid crystal molecules seem to be lying in any direction, the viewing angle is enhanced.
However, the above described IPS-LCDs have inherent problems as below.
The counter electrode 102 and the pixel electrode 106 have a selected width, for example 5 to 10 .mu.m. Although liquid crystal molecules located within 1 to 2 .mu.m from the edges of the electrodes 102, 106 are affected by electric fields, liquid crystal molecules located at the upper center of the electrodes are not, due to an equipotential surface generated at the upper center portion of the electrodes 102, 106. Therefore, the liquid crystal molecules located around the upper center portion of the electrode 102, 106 cannot be driven.
In order to resolve the above problem, there is proposed a use of a structure to cover the pixel electrode 106 and the counter electrode 102 with an opaque metal film.
The proposed structure has, however, a shortcoming that the transmittance of the LCD is reduced since the opaque pixel electrode 106 and counter electrode 102 does not pass the light from the back light.
Moreover, the aperture ratio is low due to the opaque counter electrode 102 and pixel electrode 106. Therefore, to obtain adequate brightness, a back light with high brightness is necessary, which increases power consumption.
Meanwhile, the pixel electrode 106 divides the space surrounded by the counter electrode 102 into two parts. At this time, the pixel electrode 106 is separated in the Y-direction from the counter electrode 102 by 7 to 10 .mu.m. Hence, the area where a field is created between the counter electrode 102 and the pixel electrode 106 is as large as the separated distance therebetween. However, response speed is reduced as the area of influence of field described above increases.